A. Field of the Invention
The present invention relates generally to wireless communications networks and, more particularly, to receiver scheduling in ad hoc wireless communications networks.
B. Description of the Related Art
Conventionally, an ad hoc wireless network includes a variable number of communications nodes, each node being equipped with a communications transceiver for conveying information messages throughout the network. While some nodes in the network are capable of performing network routing functions, other nodes serve exclusively as sources or destinations for data traffic. These two types of nodes are commonly referred to, respectively, as routers and endpoints. During network operation, these routers and endpoints execute predefined algorithms and adhere to established networking protocols, which together enable the nodes to find one another, determine preferred paths through the network for data traffic, and detect and repair ruptures in the network as communication path characteristics change over time, or as nodes move, fail, experience changes in battery power, etc.
Certain ad hoc wireless networks employ a scheme known as receiver scheduling to conserve power in some or all of the network nodes, or to make it more difficult for adversaries (e.g., hackers in a civilian context, or enemies in a military context) to monitor and disrupt network operation. In such receiver scheduling schemes, node receivers are selectively and intermittently turned off for some fraction of the total time that the overall network is operational. Consequently, each node is available and operating to receive information messages only during prescribed periods, and some form of receiver scheduling is employed to ensure that the various network nodes are in agreement as to when they can transmit information messages to one another.
Conventionally, receiver scheduling is accomplished using some form of Time Division Multiple Access, or TDMA, plan. While there are many variants of TDMA receiver scheduling, all such schemes share a common basic principle. Specifically, such TDMA plans divide network operating time into a number of predefined time slots and, during each time slot, certain network nodes are appointed to power up their receivers to accept incoming information messages. By sharing a common reference clock, as well as a common receiver appointment schedule, all of the nodes in the network are informed as to when they are able to successfully transmit messages to neighboring nodes.
Typically, all of the nodes in such a network are preconfigured with a single, network-encompassing TDMA receiver schedule. Alternatively, each node can transmit its own particular TDMA receiver schedule to other nodes during network operation. For example, a node can generate its own TDMA receiver schedule at run time using a pseudo-random number generator, or PRNG, and then transmit a corresponding PRNG seed to neighboring nodes (since, assuming all nodes are in agreement as to how to construct a TDMA receiver schedule from a PRNG stream, a single PRNG seed unambiguously defines a unique TDMA receiver schedule).
While these methods do achieve the stated goal of providing receiver scheduling in ad hoc wireless networks, they also have certain drawbacks. For example, in each of the above mentioned TDMA schemes, the start times and durations of the TDMA time slots are fixed. Consequently, each node is restricted with respect to the start times and durations of its reception intervals. Such rigidity inevitably results in inefficiencies, as message transmission intervals are not always precise multiples of a time slot, and thus portions of time slots sometimes go unused. Moreover, the fixed nature of the time slots can aid adversaries in attempts to monitor and disrupt network operation (i.e., the start times and durations of the time slots are not variables with which an adversary must contend). As a result, a need exists for a more flexible, efficient, and robust receiver scheduling scheme.